Airway hypoxia of the lung resulting from high altitude exposure and a number of cardiopulmonary diseases leads to pulmonary vascular constriction, pulmonary hypertension and sometimes, right heart failure. This response to hypoxia which occurs in most mammalian species including man, is particularly pronounced in cattle and also marked in swine. This has served as the impetus for our efforts to develop both cattle and swine models of spontaneous pulmonary hypertension through selective breeding. We have achieved this goal in cattle and it appears we have also developed it in swine. The major objectives of the study in the two groups of cattle have been and still are to determine the basis for their different pulmonary pressor responses to hypoxia and to add information that might be helpful in providing a better understanding of the mechanisms controlling the pulmonary circulation. During the coming year we will continue: (a) to examine by embryo transfer and progency testing whether the traits for susceptibility and resistance to hypoxic pulmonary hypertension in cattle are genetically transmitted or acquired; (b) to continue to develop through further selective breeding and progeny testing a miniature swine model of spontaneous hypoxic pulmonary hypertension; (c) to carry out morphometric studies on the pulmonary vasculature during the early post-natal periods; and (d) to determine the ventilatory changes in neonatal calves during the first week postpartum.